Japanese Pat. application No. 251,589/86 corresponding to U.S. Pat. application Ser. No. 07/312,899, U.S. No. 4,892,048 discloses a sewing machine which supplies automatically an upper thread of an amount required to form stitches by an intermittently operating presser device arranged in an upper thread running path between a thread tension device and a take-up lever and is operated in a predetermined phase of stitching.
In conventional sewing machines such as disclosed in the application Ser. No. 312,899, it was difficult to exactly cope with various elongation characteristics of various types of threads and differences in thickness of fabrics. Therefore, the elongations were dealt with by G.C.M. (greatest common measure). Namely, standard types of threads, fabric, and needle were employed to tentatively sew zigzag and/or straight stitches. If the stitches so formed showed no interconnection of upper and lower threads on the upper and lower face of a fabric, the sewing machine has been considered practically good.
U.S. Pat. application Ser. No. 07/209,856 pending discloses an automatic thread tension control method for a sewing machine, wherein a conventional thread tension device and an intermittently operated thread clamping or holding device are used in combination. As it is usual, the thread tension device is adjusted in accordance with the stitching conditions including a type of patterns to be stitched, the thickness of the fabric, a fabric feeding amount, etc. The thread tension device is adjusted and may be set with a presser to a cotton thread and to another kind of the thread such as a polyester thread. In this case, the thread elongations may be observed in the cotton threads and polyester thread, respectively, when a thread take-up lever comes to the upper dead point thereof. Such phenomenon will cause the formation of the shrunk seam. It is therefore required that the amount of the thread corresponding to the elongation, that is, a lacked amount is supplied from the thread source. Moreover it is required that such difference of the elongation depending upon the kind of the threads be compensated by controlled supply of the thread. In U.S. Pat. application Ser. No. 07/209,856 a thread damping device is provided for these purposes.